1. Field of the Invention
This invention relates to a heat-sensitive recording material which is superior in light resistance, weather resistance, oil resistance, and optical readability in the near infrared region.
2. Prior Art
In general, a heat-sensitive recording sheet is produced by applying a support, such as paper, synthetic paper, film, plastic, etc., the coating which is prepared by individually grinding and dispersing colorless chromogenic dyestuff and an organic color-developing agent, such as phenolic material, etc. into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The coating, when heated by thermal per, thermal head, hot stamp, laser beam, etc., undergoes instantaneously a chemical reaction which forms a color. These heat-senstive recording sheets have now been finding a wide range of applications, including industrial measurement recording instruments, terminal printers of computer, facsimile equipments, automatic ticket vending machines, printer for bar-code-label, and so on. In recent years, as the applications of such recording is diversified and the performance of such recording equipment is enhanced, high qualities are required for heat-sensitive recording sheets. For example, even with small heat energy in a high speed recoding, both the clear image with a high density and the better preservability such as better resistance to light, weather and oil, etc. are required. These heat-sensitive recording sheets are also utilized as thermosensitive labels. Since, however, color formation in these recording sheets is in the visible region, they cannot be adapted for reading by a semiconductor laser in the near infrared region which is widely used as a bar code scanner in a POS system, etc.
On the other hand, Japanese Laid-Open Patent Publication Nos. 59-199757 and 60-226871 disclose a heat-sensitive recording sheet containing a combination of a conventional color developing agent (phenolic resin, hydroxybenzoate and bisphenol-A) a fluorene-type leuco dyestuff having excellent color-developing ability in the near infrared region.
However, these heat-sensitive recording sheets have a remarkably inferior stability (inferior resistance to light, weather and oils) of the recorded image. Hence, in long storage under condition of exposure to light, moisture, etc., the recorded image is discolored, the image density is reduced, and sometimes the image disappears, which deteriorates a optical readability in near infrared region. By the adhesion of serum, or by contact with plasticizer (DOP, DOA etc.) in wraping film, such as vinyl chloride film, the image density is prominently lowered or the recorded image disappears, which causes a remarkable reduction of a optical readability in the near infrared region. In view of above defects, the practical use of these heat-sensitive recording sheets was difficult.